Electrical characterization of dielectric thin films for MEMS application

S. Motahari

    Research output: ThesisEngD Thesis


    Capacitive Micromachined Ultrasonic Transducer (cMUT) is being fabricated at Philips research. The final goal of this MEMS-based device, in which a vibrating membrane with electrodes is responsible for electroacoustic transduction, is to generate and detect ultrasonic waves.
    Ultrasound is a medical imaging technique used to provide critical information from inside a patient’s body for many medical applications.
    Among different components of the cMUT, the dielectric layer is the most sensitive part for its performance, yield and reliability. Dielectric charging and breakdown in this layer are among the highest cMUT related reliability risks. It is for this reason that dielectric improvement plays an important role for the lifetime of the final device.
    To assess the electrical properties of these dielectric layers, several experimental techniques have been used to determine dielectric constant, breakdown field, charge to breakdown and leakage current density.
    To optimize the layer and its structure, test capacitors with different bulk dielectric materials as well as different electrodes have been fabricated and mentioned parameters of the dielectric thin films were measured electrically.
    The project is divided into two phases. The first phase is focused on how to improve the SiO2 dielectric quality (both bulk and interface). The second phase is to replace the current dielectric (SiO2) with a new material that has a higher dielectric constant (‘high‐k’). High‐k dielectrics allow the use of lower voltages without sacrificing the acoustic properties of the cMUT. In this report, Al2O3 is considered as the high-k candidate to replace SiO2.
    In summary, SiO2 dielectric layers deposited with Atomic Layer Deposition (ALD) show higher lifetimes compared to SiO2 layers deposited with the more conventional technique of plasma-enhanced chemical vapor deposition (PECVD).
    For a reasonable good dielectric, interface is shown to be as important as bulk and electrical performance of dielectric is sensitive to the structural properties of both bulk and interface and their deposition method.
    Al2O3 has higher dielectric constant, but lower breakdown field and higher leakage current density, which makes it undesirable for cMUT application. However, multi-layer stacks including Al2O3 and SiO2 layers grown by atomic layer deposition, are closer to the optimum stack for cMUT application.
    Moreover among various multi-layer stacks including both ALD SiO2 and ALD Al2O3, the one with highest portion of SiO2 (%) shows to be the most promising candidate for cMUT application in terms of leakage current density, lifetime, critical field and breakdown field, the typical values being 1 nA/cm2, 10 years, 2.5 MV/cm and 7 MV/cm respectively.
    With the current design and materials, there is a trade-off between dielectric constant and other desired parameters in multi-layer stacks containing ALD Al2O3 and ALD SiO2. Multi-layer stacks with other high-k materials (i.e. HfO2) could be engineered in future designs in order to satisfy the application requirements.
    Original languageEnglish
    Awarding Institution
    • Cottaar, E.J.E. (Ward), Supervisor
    • Roozeboom, F., Supervisor
    • Groenland, A., External supervisor, External person
    Award date18 Dec 2015
    Place of PublicationEindhoven
    Publication statusPublished - 2015

    Bibliographical note

    PDEng thesis. - Confidential forever.


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